Changes:
4722764c4 [vpython] Set default Python version explicitly

Bug: 1003552
Change-Id: I01c32b00b3b0ea7bb555d138e56228ae8346e50c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1801812Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: smut <smut@google.com>